Semiconductor device fabrication frequently involves patterning features through the use of a mask and photo-sensitive material. A reticle is used to print images onto a mask, which is used to form patterns on the photo-sensitive material. Some reticles, called dark field reticles, are mainly chrome with features that are opened up where light is transmitted. Other reticles, called bright field reticles, are mainly glass with features defined by pieces of chrome.
Due to constraints in the lithographic process, the pattern formed in the photo-sensitive material does not coincide exactly with the mask pattern. Conventional masks often compensate for this phenomenon including features that differ somewhat from the features desired to be patterned in the photo-sensitive material.
For example, isolated main features will almost always print at a feature size significantly different from the same mask feature surrounded by other features. This is known as iso-dense bias. A main feature is a location on the reticle where a permanent feature should be printed and etched on a mask that corresponds as closely as possible to the original feature drawn by the designer. A main feature is typically a type of polygon.
To correct iso-dense bias, sub-resolution assist features (SRAFs), also known as scattering bars, are added to the mask. The SRAFs are in the shape of a polygon and are designed to make an isolated feature seem denser, and therefore, allow the isolated feature to print at the same feature size as a dense feature. An SRAF can be both positive and negative tone on the reticle. For example, on a bright field reticle, an SRAF can be defined by a piece of chrome and also by glass; when a piece of chrome defines a main feature, a glass opening can be cut into that chrome to define an SRAF.
The SRAF is a sub-resolution feature and, therefore, is not meant to print. It is carefully adjusted in size so that it never prints over the needed process window. Thus, SRAFs are designed to make the assist features as large as possible to create a denser mask pattern, but not so large as to print.
After adding SRAFs, optical proximity correction (OPC) is typically run-on the mask layout before the mask is fabricated. OPC is the process of modifying the polygons on the mask pattern to compensate for the non-ideal properties of the lithography process. If SRAFs are not positioned properly to eliminate iso-dense bias after OPC has been run, the pattern formed in the photo-sensitive material will not correctly correspond to the pattern in the mask.
Accordingly, it is desirable to provide systems and methods for checking the mask layout to determine if the SRAFs have been positioned and sized properly to eliminate iso-dense bias.